Our goal is the development of methods for the specific isolation of antigen-reactive cells (ARC) in order to study the molecular interactions that occur in their immune reactions. Populations of cells containing ARC are tested for binding to the cell surface antigens of target cells when the target cells and/or their antigens have been attached to insoluble surfaces. Separated ARC populations are tested for cytotoxic effector cells (CTL) and their precursors, and for activity in allograft rejection and graft-versus-host and mixed lymphocyte reactions. T cell subpopulations are separated and characterized by reactions with specific reagents such as monoclonal antibodies to cell surface antigens by flow cytometry. Surface molecules of target cells, in particular class I MHC antigens, are isolated to test their binding to ARC.